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AD637BR Datasheet(PDF) 7 Page - Analog Devices |
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AD637BR Datasheet(HTML) 7 Page - Analog Devices |
7 / 10 page REV. E –7– AD637 Table I. Practical Values of CAV and C2 for Various Input Waveforms Input Waveform and Period Absolute Value Circuit Waveform and Period Minimum R CAV Time Constant Recommended CAV and C2 Values for 1% Averaging Error@60Hz with T = 16.6ms Recommended Standard Value CAV Recommended Standard Value C2 1% Settling Time Symmetrical Sine Wave Sine Wave with dc Offset Pulse Train Waveform 1/2T T A B C D 181ms 325ms 2.67sec 2.17sec 1.5 F 2.7 F 22 F 18 F 6.8 F 0.82 F 0.47 F 5.6 F 10(T – T2) 10(T – 2T2) 1/2T T2 T2 T2 T2 0V 0V 0V 0V T T T T T T T FREQUENCY RESPONSE The frequency response of the AD637 at various signal levels is shown in Figure 10. The dashed lines show the upper frequency limits for 1%, 10% and ±3 dB of additional error. For example, note that for 1% additional error with a 2 V rms input the high- est frequency allowable is 200 kHz. A 200 mV signal can be measured with 1% error at signal frequencies up to 100 kHz. INPUT FREQUENCY – Hz 10 1k 10M 10k 100k 1M 1 0.1 0.01 1V RMS INPUT 2V RMS INPUT 7V RMS INPUT 100mV RMS INPUT 10mV RMS INPUT 1% 3dB 10% Figure 10. Frequency Response To take full advantage of the wide bandwidth of the AD637 care must be taken in the selection of the input buffer amplifier. To insure that the input signal is accurately presented to the con- verter, the input buffer must have a –3 dB bandwidth that is wider than that of the AD637. A point that should not be over- looked is the importance of slew rate in this application. For example, the minimum slew rate required for a 1 V rms 5 MHz sine-wave input signal is 44 V/ µs. The user is cautioned that this is the minimum rising or falling slew rate and that care must be exercised in the selection of the buffer amplifier as some amplifi- ers exhibit a two-to-one difference between rising and falling slew rates. The AD845 is recommended as a precision input buffer. AC MEASUREMENT ACCURACY AND CREST FACTOR Crest factor is often overlooked in determining the accuracy of an ac measurement. Crest factor is defined as the ratio of the peak signal amplitude to the rms value of the signal (C.F. = Vp/ V rms). Most common waveforms, such as sine and triangle waves, have relatively low crest factors ( ≤2). Waveforms which resemble low duty cycle pulse trains, such as those occurring in switching power supplies and SCR circuits, have high crest factors. For example, a rectangular pulse train with a 1% duty cycle has a crest factor of 10 (C.F. = 1 η ). PULSEWIDTH – s 10 1.0 0.01 1 1000 10 100 0.1 CAV = 22 F CF = 10 CF = 3 0 100 F Vp T e0 = DUTY CYCLE = 100 s T CF = 1/ eIN (rms) = 1 Volt rms Figure 11. AD637 Error vs. Pulsewidth Rectangular Pulse Figure 12 is a curve of additional reading error for the AD637 for a 1 volt rms input signal with crest factors from 1 to 11. A rectangular pulse train (pulsewidth 100 µs) was used for this test since it is the worst-case waveform for rms measurement (all CREST FACTOR +1.5 0 –1.5 111 2 3456 7 8 9 10 +1.0 +0.5 +0.5 –1.0 POSITIVE INPUT PULSE CAV = 22 F Figure 12. Additional Error vs. Crest Factor |
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